1. Field of tile Invention
The present invention relates to a zinc alloy powder for use in an alkaline cell and a method to produce tile same. More particularly, the present invention is relates to a non-amalgamated zinc alloy powder for use in an alkaline cell, which comprises zinc containing iron in an amount of not more than 1 ppm as an inevitably accidental Impurity, and specific elements added so as to suppress the evolution of hydrogen gas and to Improve the leaktightness of a cell without the use of mercury and lead which are toxic elements, and a method to produce the same.
2. Prior Art
The mercury contained in an amalgamated zinc powder used as an anode active material in an alkaline cell has been known to be an essential component for such an active material from the viewpoint of suppressing the evolution of hydrogen gas due to the corrosion of zinc and preventing a liquid from leaking From the cell as a result of the evolution of hydrogen gas.
In light of environmental protection, however, a reduction in the mercury content is required in this field. In line with this requirement, it has become possible to suppress the evolution of hydrogen gas through the addition of not only lead but also aluminum, bismuth, indium and the like as additional elements to zinc so that the mercury content is remarkably reduced from 10% by weight to about 1% by weight.
As further social needs, in recent years, it is required to decrease the mercury content of the anode active material to 0% by weight, in other words, to effect non-amalgamation. This non-amalgamation greatly changes the situation, and it has been difficult to decrease the evolution of hydrogen gas to a desired level even when the above-described additional elements are added. That is, although zinc alloy powders as an anode active material having various types of elements added thereto have been proposed {see, for example, Japanese Patent Appln. Publication Gazette No. (Hei.) 2-22984 (22984/1990) and Japanese Patent Appln. Laid-Open Gazette No. (Sho.) 61-53950 (153950/1986)}, it has been impossible to attain the desired suppression of hydrogen gas evolution when the mercury content is 0% by weight, though such suppression has been possible even when the mercury content is 1% by weight or less.
Recently, in a trend to minimize the content of mercury, the effect of lead to inhibit the zinc corrosion has become increasingly important. Accordingly, anode active materials of low-mercury alkaline cells which have been commercially available generally consist of alloy compositions such as zinc-lead, zinc-aluminum-lead, zinc-aluminum-indium-lead and zinc-bismuth-lead. It has been generally believed that the minimization of the content of mercury is largely attributed to the effect of lead being added, and that non-amalgamation of the anode active material can never be achieved if the lead is not used at all.
By the way, it is known that lead is also as harmful to the human body as mercury is. Accordingly, in view of social demand for a clean environment, the intentional addition of the lead is also not desirable. As mentioned above, however, it has not been possible to date to realize the production of a lead-free active anode material, even in the case of the low amalgamation.
Meanwhile, attempts have been made to suppress the evolution of hydrogen gas and to improve the discharge performance by reducing the impurity content of zinc. For example, Japanese Patent Appln. Laid-Open Gazette No. (Sho.) 62-123653 (123653/1987) describes a reduction in the content of impurities such as iron and chromium. Table 1 on page 4 of the published specification shows that an improvement in the discharge performance while suppressing the evolution of hydrogen gas is attained by reducing the iron content to about 10 ppm in an anode active material which comprises an amalgamated zinc alloy powder containing predetermined amounts of lead, indium and aluminum and containing 1% by weight of mercury.
However, a zinc alloy powder having a mercury content of 0% by weight could not attain the desired effect of suppressing the evolution of hydrogen gas even when the content of iron contained as an impurity was reduced to about 10 ppm and additional elements such as lead were incorporated.
Thus the non-amalgamation of an anode active material and the freeing the material from lead are accompanied by a difficulty which is fundamentally different from that encountered in the low amalgamation leading to a mercury content of 0.6 to 1% by weight, and there has not been developed any alkaline cell wherein a non-amalgamated and lead-free zinc alloy powder is used as an anode active material, the evolution of tile hydrogen gas is suppressed, and the leaktightness is improved.